1. Field of the Invention
This invention relates generally to a procedure for coating prosthetic alloys with a bone-emulating substance and, more particularly, to electro-stimulated phosphate (brushite) growth on metallic conductive substrates.
2. Background Information
Although the mechanism by which fixation of bone to a transplant occurs is somewhat beyond the scope of the instant disclosure, is has been observed that the coating of metallic prostheses with phosphate ceramics has received a great deal of recent attention because of the apparent propensity of these coatings to accelerate bone fixation during the early stages following implantation. Current articles in the literature have given ample reason to believe that the rate of metal ion release from some alloys can be reduced by calcium phosphate coatings. Further, reviews concerning applications of hydroxyapatite coatings on metallic implants were given significant treatment by Ducheyne, P.; Lemons, J. E.; Eds.; "Bioceramics: Material Characteristics Versus In Vivo Behavior", New York, The New York, Academy of Sciences, 1988.
The most frequently used means or process for the deposition of calcium phosphate materials on prosthetic alloys is by way of plasma or flame spraying. There was recently reported by Takayuki Shimamune and Masashi Hosonuma in Chemical Abstracts, volume 109:11784d and volume 109:11785e, data on a calcium phosphate-coated medical composite implant material and a process for its manufacture. Therein, it is indicated that a calcium phosphate-coated compatible composite material, comprising a metallic substrate and an oxide layer of more than one metal selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, tin, cobalt, aluminum, chromium, molybdenum and tungsten is overlayed with a layer of calcium phosphate which can be produced by plasma or flame spraying. This composite has an affinity for the tissue of bone or teeth and finds its use as an implant material for artificial bone, teeth and teeth roots, or as a bonding material for such implant materials.
Also significant, and as disclosed in the Chemical Abstracts references are European Patent applications 0,264,353 and 0,264,354, both made by Shimamune and Hosonuma. These disclosures teach a composite material and a process for the production thereof which comprises a metallic substrate having thereon an oxide layer, the oxide layer consisting essentially of the oxide of one or more metals denoted in the previously mentioned group, and thereafter a calcium phosphate overlay on the oxide layer. The composite is made by oxidizing a metallic substrate, either thermally or electrolytically, to form a layer of the oxide or the metallic substrate component alone or a layer of mixed oxide of the metallic substrate component and a metal component in the electrolyte. Alternatively, heating of the metallic substrate is accomplished to stabilize the surface thereof; and, then a coating of calcium phosphate compound is formed on the surface. Essentially, the Shimanune et al methodology comprises two distinct processes. The first, the perhaps preferred embodiment, is a process used to produce a calcium phosphate compound-coated composite material suitable as an implant material. This comprises thermally oxidizing a metallic substrate to form, on the surface of the metallic substrate, a layer of the oxide of the metallic substrate component, such providing excellent corrosion resistance in the living body. Then, there is formed on the oxide coating a layer of calcium phosphate compound such as apatite hydroxide, which has been determined to have affinity to the living body on the surface of the aforementioned oxide layer. The second Shimamune et al. embodiment is a process for producing a calcium phosphate compound-coated composite material, also suitable as an implant, which comprises electrolizing a metallic substrate in an electrolyte to form on the surface thereof a coating of the oxide of the metallic substrate component alone or a mixed oxide of the metallic substrate component and a metal component of the electrolyte. Thereafter, as in the first embodiment, there is formed on the oxide, a calcium phosphate coating such as apatite hydroxide.
When the metallic substrate is made of stainless steel or a cobalt-chromium alloy, unlike the case wherein the metallic substrate is made of titanium or a titanium alloy, it is necessary to become highly selective in the electrolyte usage. Shimamune et al. teach that, if anode polarization is carried out in an acidic solution, the metal surface is dissolved and the desired oxide layer becomes difficult to obtain. Continuing, they teach that, in a strongly alkaline solution, the oxide on the surface of the metallic substrate is partially dissolved and thus, in some cases, a sufficiently grown oxide layer cannot be obtained. The limitation which is therefore placed on the process is that it becomes necessary to choose an electrolyte having a pH of 6 to 13. Quite matter-of-factly, Shimamune et al. insist that the method of forming the calcium phosphate coating and the conditions under which the method is carried out are not critical.
The proposition that an electrolytic method for phosphate deposition on an alloy serving as the anode is not only commonplace, but also the current state-of-the-art, finds substance in the issuance of a patent in 1985 to Shindow et al. (U.S. Pat. No. 4,522,892). Shindow et al. teach a steel strip having phosphate-coating property produced by subjecting at least one surface of a steel strip to electrolytic treatment in which the strip serves as an anode. Its surface is brought into contact with an aqueous solution containing at least one phosphate selected from the group consisting of alkali metal phosphates and ammonium phosphate; the solution having a concentration of phosphoric anions of 0.05 mole/L or more and a pH of from 4 to 7. Thus, the noteworthy factors in the Shindow process are the electrolytic formation of the coating on an anode at a pH between 4 and 7. In contrast to the procedure I have developed, it is particularly noteworthy that the process of Shindow et al. is an oxidation which occurs in an electrolyte solution that does not contain the cation to be deposited. The contention that an electrophoretic method of phosphate deposition on a metal serving as the cathode reflects the current state-of-the-art, finds substance in the issuance of a patent in 1989 to Hemminger et al. (U.S. Pat. No. 4,806,218). Therein, electrophoresis is termed cataphoretic because the final coating is placed on the tungsten element or wires while they act as a cathode. Hemminger et al. avoid a certain degree of erosion of the electrode by first applying polarity and treating it as an anode for the purposes of incipient coating; thereafter, the polarity is reversed and the element to be coated spends the duration of the electrophoretic processing period as a cathode. As in the previous teachings, it is the purpose of these patentees to place an oxide coating on the electrodes and, therefore, the electrolyte is generally devoid of phosphate ions. Hemminger et al. differ from Shindow et al. in that the electrode of interest is a cathode which is used electrophoretically, while Shindow uses the electrode of interest as an anode in an electrolytic cell (as was the case in the procedure of Shimamune et al.).
Electrophoretic deposition of phosphate materials has recently received a good deal of professional attention. It is seen that, in most conventional and state-of-the-art coating processes, particles are suspended in a liquid and, in the presence of a large electric field, are driven onto an electrode. Analysis of these coatings, after sintering, indicates the presence of hydroxyapatite and tricalcium phosphate. These techniques are particularly attractive because irregularly shaped substrates, such as are employed in the various prostheses, can be coated conveniently and relatively inexpensively.
Generally speaking, electrolytic deposition methods should grow to greater popularity since most of the heretofore conventional methods of preparing implants with phosphate coatings have employed flame spraying and plasma spraying. With these latter procedures, heating of the substrate surface can be extensive and thermal decomposition of the material being sprayed is often observed. Other high temperature techniques, such as dip coating and sputter coating, can also suffer from thermal degradation, inconveniences and disabilities which make electrolytic deposition a superior coating procedure.